This class represents an abstract model backend to the IMAP server; it it meant to be overridden by server implementations.
Primarily,
subclasses are expected to override "init" to set up their folder structure.

Called when the class is instantiated,
with no arguments.
Subclasses should override this method to inspect the "auth" object,
and determine what folders the user should have.
The primary purpose of this method is to set "root" to the top level of the mailbox tree.
The root is expected to contain a mailbox named INBOX.

Gets or sets the root Net::IMAP::Server::Mailbox for this model.
The root mailbox should contain no messages,
and have no name -- it exists purely to contain sub-mailboxes,
like INBOX.
The "init" method is responsible for setting up the appropriate root mailbox,
and all sub-mailboxes for the model.

Utility method which splits a given PATH according to the mailbox separator,
as determined by the "separator" in Net::IMAP::Server::Mailbox of the "root".
May die if the path (which is expected to be encoded using IMAP-UTF-7) is invalid.
See Encode::IMAPUTF7.
If the mailbox hierarchy is flat (i.e.
the separator is undef),
returns the name without change.